ELECTRICAL. STRUCTURE OF MATTER RUTHERFORD 181 



hility. In some respects an " excited " atom of this kind is thus 

 analogous to a radioactive atom, but of course the energy released 

 in the disintegration of a nucleus is of an entirely different order of 

 magnitude from the energy released by return of the electron in the 

 excited atom. It may be that the elements, uranium and thorium, 

 represent the sole survivals in the earth to-day of types of elements 

 that were common in the long-distant ages, when the atoms now com- 

 jiosing the earth were in course of formation. A fraction of the 

 atoms of uranium and thorium formed at that time has survived 

 over the long interval on account of their very slow rate of trans- 

 formation. It is thus possible to regard these atoms as having not 

 yet completed the cycle of changes which the ordinary atoms have 

 long since passed through, and that the atoms are still in the " ex- 

 cited '' state where the -nuclear units have not yet arranged them- 

 selves in positions of ultimate equilibrium, but still have a surplus 

 of energy which can only be released in the form of the character- 

 istic radiation from active matter. On such a view, the presence of 

 a store of energy ready for release is not a property of all atoms, but 

 only of a special class of atoms like the radioactive atoms which have 

 not yet reached the final state for equilibrium. 



It may be urged that the artificial disintegration of certain ele- 

 ments by bombardment with swift a particles gives definite evidence 

 of a store of energy in some of the ordinary elements, for it is Icnown 

 that a few of the hydrogen nuclei, released from aluminium, for 

 example, are expelled with such swiftness that the particle has a 

 greater individual energy than the a particle which causes their 

 liberation. Unfortunately, it is very difficult to give a definite 

 answer on this point until we know more of the details of this dis- 

 integration. 



On the other hand, another method of attack on this question has 

 become important during the last few years, based on the compari- 

 son of the relative masses of the elements. This new point of view 

 can best be illustrated by a comparison of the atomic masses of 

 hydrogen and helium. As we have seen, it seems very probable that 

 helium is not an ultimate unit in the structure of nuclei but is a 

 very close combination of four hydrogen nuclei and two electrons. 

 The mass of the helium nucleus, 4.00 in terms of 0=16, is con- 

 siderably less than the mass 4.03 of four hydrogen nuclei. On mod- 

 ern views there is believed to be a very close connection between 

 mass and energy, and this loss in mas? in the synthesis of the helium 

 nucleus from hydrogen nuclei indicates that a large amount of 

 energy in the form of radiation has been released in the building 

 of the helium nucleus from its components. It is easy to calcu- 

 late from this loss of mass that the energy set free in forming 1 

 20397—25 13 



